The present invention relates to a method and system for compensating for signal propagation delay between network nodes in a data communications network.
More particularly, the present invention relates to a method and system for compensating for signal propagation delay which can be integrated with a method or system for controlling network node transmit power levels, and which is of particular use in a data communications network where accurate power control and time synchronisation is required.
Data networks can be classified in many ways, but for the purpose of the present invention, it is useful to classify them by their means of accessing the medium over which data is communicated. The relevant classifications are broadcast and non-broadcast.
An existing type of data network is Ethernet. Ethernet uses broadcast medium access. All network nodes sharing the network medium hear all traffic being passed over the medium. Traffic is directed to individual network nodes via physical layer addresses that are attached to the data packets being sent over the medium. When multiple network nodes attempt to transmit data simultaneously, there is the possibility for contention among the nodes for access to the medium.
A modification to the broadcast network is the broadcast network with hidden terminals. In this network, all terminals share the same medium, however it cannot be guaranteed that all terminals can hear each other. All that can be guaranteed is that all terminals can hear the central network node, referred to herein as the access point. For this reason, it is not enough for each terminal simply to monitor the channel in order to detect contentions. Feedback on success or failure of network contention must also be communicated back to the network terminals by the access point.
Point-to-point full duplex connections, in contrast to the above, do not require contention for the network medium. This is because only two network elements, at each end of the network medium, share access to the medium.
Point-to-multipoint networks in which several network nodes share access to a network medium can simulate point-to-point connections. In such a network a fixed time slot is assigned to each node of the network, and the transmissions of each node are restricted to its particular assigned time slot. An example of such an arrangement of the prior art would be a Time Division Multiple Access (TDMA) network. A cellular wireless network, in which there is a central access point and multiple subscriber terminals sharing access to the channel using TDMA medium access, is an example of such a network. In such networks, since the various subscriber terminals are separated from the central access point by an unknown distance, there is a bounded but unknown elapsed time of propagation as the signal passes between the subscriber terminal and the central access point. In this situation, a guard time equal to twice the maximum propagation time over the cell radius must separate each time slot, since the network has no knowledge from where in space each burst transmitted in a time slot will originate. The problem will be illustrated further with reference to FIGS. 1 and 2.
Within FIG. 1, a central access point 2 provides access to a wide area network (not shown) for a number of subscriber terminals 4. The subscriber terminals may be scattered throughout the access point cell coverage area. The cell coverage area may further be split into sectors 3 and 5 wherein each sector is covered by a different frequency. Now consider one sector containing subscriber terminals 6, 8, and 10, each respectively further from the access point than the former. With reference to FIG. 2, it is apparent that each particular upstream time slot 20 must have allotted an amount of time corresponding to the sum of a guard time 21 and a burst time 23. The first time slot 20 follows immediately from the downstream burst 25, allowing for RF turnaround time 27. As the access point does not know how far away the terminal which has been allotted that particular transmission slot is, the guard time must be provided to allow for the maximum signal propagation delay across the cell. For example, if time slot 20 has been allotted to terminal 6, then the burst 14 from terminal 6 begins to arrive only a little after the start of the guard time as shown by arrow 302. However, if the time slot 20 is allotted to terminal 10, then the signal propagation delay from terminal to the access point causes the upstream burst 18 from terminal 10 to begin to arrive at the end of the guard time as shown by arrow 304. Where the subsequent time slot is then allotted to a different terminal (e.g. terminal 6), there is the possibility that signals from the first time slot (i.e. from terminal 10) and the subsequent time slot (i.e. from terminal 6) could arrive at the access point concurrently, thus corrupting each signal. In order to avoid this, guard times (22) must be provided for each time slot. This clearly reduces overall transmission efficiency, as a significant portion of each upstream time slot must be vacant.
The present invention overcomes this problem by providing a method and system which compensates for the differences in signal propagation delay by causing each subscriber terminal to artificially simulate being at the same distance from the access point as every other subscriber terminal. This eliminates the need for guard times between each subsequent slot, as the propagation delays are simply forced to be the same for every subsequent transmission and hence there is no possibility of bursts transmitted in different time slots arriving at the access point at the same time. Only one guard time is required at the start of the very first upstream time slot to allow for the very first propagation delay. Subsequent time slots then do not require guard times as the delay is always the same. The removal of the requirement for guard times means that channel efficiency is improved.
According to the present invention, there is provided a method of compensating for signal propagation delay in a data communications network comprising a central control node and one or more remote subscriber nodes, comprising the steps of:
a) measuring the respective signal propagation delays for each subscriber node; and
b) using the respective signal propagation delays in each remote subscriber terminal to artificially simulate that each remote subscriber terminal is at the same distance from the central control node as every other remote subscriber terminal.
Each remote subscriber terminal may artificially simulate being at a maximum allowable distance corresponding to the maximum signal propagation delay from the central control node.
The data traffic on the network can be regulated per unit time frame by the central control node. In this case, the measuring step a) further comprises the steps of: designating a registration time slot per time frame in which the remote subscriber nodes may each first transmit a first transmission; transmitting said first transmission from the remote subscriber nodes at the start of the registration time slot; measuring one or more respective time period from the start of the registration time slot to the receipt at the central control node of each first transmission from each remote subscriber node, each time period corresponding to a respective propagation delay for transmission of signals from one of the remote subscriber nodes to the central control node; and indicating to each respective remote subscriber node the respective measured time period for that node; wherein each remote subscriber node then uses its respective measured time period to compensate for signal propagation delay when transmitting subsequent data traffic to the central control node.
The designating step may further comprise the steps of: defining said registration time slot for a present time frame at said central control node; and transmitting a control data portion to each remote subscriber node from the central control node, said control data portion including an indication of when said registration time slot for the present time frame is to occur.
The measuring step may further comprise the steps of: setting a value in a countdown timer, said value corresponding to a minimum signal propagation delay; starting said countdown timer to count down from said value at the start of the registration time slot; and reading the value from said timer at the moment when each respective first transmission from the respective remote subscriber node is received at the central control node; wherein said read values are then directly indicated to the respective remote subscriber nodes as said time periods.
The remote subscriber terminals may use the read values to compensate for delay by delaying any subsequent transmission of data traffic by an amount of time corresponding to the read value.
The method of the present invention may be used in combination with a method of open-loop power control of the transmit power of the remote subscriber terminals, as described later.
The method of the present invention may also be used in combination with a method of baseband delay compensation, as described later.
According to another aspect of the present invention, there is provided a system for compensating for signal propagation delay in a data communications network comprising a central control node and one or more remote subscriber nodes, said system comprising:
a) measuring means for measuring the respective signal propagation delays of each subscriber node; and
b) means for using the respective signal propagation delays in each remote subscriber terminal to artificially simulate that each remote subscriber terminal is the same distance from the central control node as every other subscriber terminal.
In the system, data traffic on the network may be regulated per unit time frame by the central control node, and the measuring means may comprise: control means for designating a registration time slot per time frame in which the remote subscriber nodes may each further transmit a first transmission; transmission means for transmitting said first transmission from each of the remote subscriber nodes at the start of the registration time slot; counter means for measuring one or more respective time periods from the start of the registration time slot to the receipt at the central control node of each first transmission from each remote subscriber node, each time period corresponding to a respective propagation delay for transmission of signals from one of the remote subscriber nodes to the central control node; and means for indicating to each remote subscriber node the respective measured time period for that node; wherein each remote subscriber node further includes means for using its respective measured time period to compensate for signal propagation delay when transmitting subsequent data to the central control node.
The control means may further comprise: means at each central control node for defining said registration time slot for a present time frame; and means for transmitting a control data portion to each remote subscriber node from the central control node, said control data portion including an indication of when said registration time slot for the present time frame is to occur.
The counter means may further comprise: means for setting a value in a countdown timer corresponding to a minimum signal propagation delay; means for starting said countdown timer to count down from said value at the start of the registration time slot; and means for reading the value from said timer at the moment when each respective first transmission from each respective remote subscriber node is received at the central control node; wherein said read values are then directly indicated to each respective remote subscriber nodes as said time periods.
Each subscriber terminal may use its respective read value to compensate for signal propagation delay by delaying any subsequent transmission of data traffic by an amount corresponding to the respective read value.
The system of the present invention may be used in combination with a system for open-loop power control of the transmit power of the remote subscriber terminals, as described later.
The system of the present invention may also be used in combination with a system for baseband delay compensation, as described later.
It is an advantage of the present invention that time delay compensation is achieved without using any additional network bandwidth. The control loop is also open, and as such has no unwanted dynamics or transients.
It is a further advantage of the present invention that the time delay compensation of the present invention maximizes the network""s bandwidth efficiency, since the guard times between upstream bursts can be eliminated.